This invention relates to optical spectroscopy, and particularly to focusing accessories for use in directing the radiation to, and receiving the radiation from, a sample under analysis.
Such accessories may conveniently be considered in two general categories. One category (see Doyle Application Ser. No. 298,067, filed Aug. 31, 1981, now U.S. Pat. No. 4,473,295 and having the same assignee as this application) includes reflectance accessories in which the radiation leaving the sample holder is traveling in a different direction from the radiation reaching the sample holder, because of its reflectance by the sample.
The other category, which is of primary interest in the present application, includes accessories in which the radiation leaving the sample holder is traveling in essentially the same direction as the radiation reaching the sample holder. This category, in turn, comprises transmission-type accessories in which the radiation passes through the sample, and multiple internal reflectance (MIR) accessories in which the radiation enters at one end of a sample holder providing an enclosed reflecting path and is reflected several (or numerous) times before leaving the sample holder. The latter type is also commonly identified as attenuated total reflection (ATR).
Where the apparatus of the present invention is used for transmission-type spectroscopy, it will be of particular interest in "micro-sampling," which involves beam condensing for use with small samples, micro pellets and liquid cells. In general, larger samples are adequately handled by the basic focusing system of the spectrometer, but smaller samples require the use of shorter focal lengths in order to concentrate sufficient light at the sample. And shorter focal lengths are more likely to have optical aberration problems.
Where the apparatus of the present invention is used for MIR (or ATR) spectroscopy, it will be of particular interest because of its positional adjustability to accommodate different positions of the sample holder, and also because of its dual function as a beam condensing and an MIR accessory.
Various accessories are available for the functions under discussion. An example of a beam condenser accessory is illustrated on sheet 16 of the loose-leaf, undated catalogue of Harrick Scientific Corporation. Such accessories generally use several mirrors to adequately diverge the beam and refocus it at a smaller spot. They require a number of internal adjustments, and little free space is available for a sample holder.
An example of an internal reflection (MIR) attachment is illustrated on sheet 12 of the same catalogue of Harrick Scientific Corporation. Such a device has additional cost because it is a separate accessory from the beam condenser, and it also has the deficiency of requiring a somewhat complex and sensitive adjustment of mirror positions.
In order to provide an ideal accessory to accomplish the functions discussed above, the following features are considered important:
(1) Locating the sample so that it does not interfere with the optical path;
(2) Minimizing optical aberrations in the accessory;
(3) Using a single accessory for both beam condensing and MIR purposes;
(4) Avoiding a requirement for X, Y, or Z translational adjustments of the whole accessory;
(5) Providing adjustments which permit moving the image point without introducing aberrations;
(6) Using a minimum number of optical elements; and
(7) Reducing the criticality of the requirements for internal adjustments when the accessory is installed.
Although reflectance accessories of the type disclosed herein can be used with a wide variety of spectrometers, they are particularly suited for use with Fourier Transform Infrared (FTIR) instruments, and make use of the fact that the sample region beams in these instruments can be made available in a collimated form with a circular cross section.